1. Field of the Invention
The present invention relates to a packaged dynamoelectric machine having an enclosure. More particularly, the invention relates to a dynamoelectric machine load package having an acoustically isolated enclosure for reducing the amount of structure-borne and air-borne noise reaching the surrounding environment.
2. Description of the Prior Art
A gas turbine driven dynamoelectric machine load package includes a load package base upon which is mounted a dynamoelectric machine such as a generator, a reduction gear and associated equipment for generator and gear ventilation. Such a load package is designed to be assembled in a factory and then shipped as a unit to a site, possibly outdoors, where it will be used.
The load package base comprises a horizontal steel base plate supported on I-beams placed around the periphery thereof. Additional I-beams for supporting the base plate may be placed at various other locations under the base plate. The dynamoelectric machine includes a stator, preferably the armature winding, which includes an armature frame. The dynamoelectric machine further includes a rotor, preferably the field winding, which includes a rotatable generator shaft supported by bearing pedestals. The rotatable generator shaft is mechanically connected to be driven by the reduction gear. The reduction gear, the bearing pedestals and the armature frame are each firmly attached to the base plate. In order to preserve shaft and bearing alignment, it is essential that the reduction gear and the bearing pedestals be firmly attached to the steel base plate in steel-to-steel contact. The base plate may include a cut-out so that a portion of the armature can extend below the base plate to reduce the overall height of the load package.
The load package is adapted to mate at one end with a first similar package including a gas turbine having an output shaft mechanically connected to rotate the reduction gear and thus the generator shaft. The load package may be adapted to mate at the other end with a second similar package including electrical switching and control equipment. A solidly-attached enclosure for reducing the amount of noise transmitted from the load package to the surrounding environment may be placed around the load package. The enclosure, if included, is assembled to the load package at the factory and shipped with the load package. Such an enclosure is open at one end and is adapted to mate at the one end with a first similar enclosure for the first similar package including the gas turbine. Electrical bus bars may project from an opening in the other end of the enclosure for electrical connection to the electrical switching and control equipment included in the second similar package. A second similar enclosure for the second similar package is adapted to mate with the other end of the load package enclosure.
A load package containing a reduction gear and a generator produces both vibration and audible noise during operation. The vibration and noise produced is accentuated by the firm mounting of the reduction gear and of the generator shaft bearing pedestals to the steel base plate. Due to this firm mounting, vibrations are directly transmitted to the base plate which acts as a large vibrating diaphragm to transmit the vibrations as sound through the surrounding air. As discussed above, this firm mounting is required in order to preserve shaft and bearing alignment. In particular, two "noise spikes" at discrete frequencies, namely 120 Hertz and approximately 1,000 Hertz, are produced and have been found to be especially difficult to eliminate. The 120 Hertz noise spike originates within the generator and the approximately 1,000 Hertz noise spike originates within the reduction gear.
Prior art attempts using solidly-attached enclosures to reduce the structure-borne and air-borne noise produced by a reduction gear and a generator in a load package have included the following. Individual components of the enclosure and load package have all been designed to have all mechanical resonant frequencies well above or well below forcing frequencies generated by the reduction gear and the generator. In order to isolate and attenuate vibration and sound at a frequency of 120 Hertz, the armature has been acoustically isolated from the load package base either by neoprene rubber isolation pads between the armature frame and the load package base or by using a "spring bar mounting" to isolate the armature from its frame and firmly attaching the armature frame to the load package base. A spring bar mounting is a mounting in which the armature is hung on a plurality of relatively long steel bars, the upper ends of the steel bars being attached to spring bars located near the top of and attached to the armature frame. Gear shaft torsional dampers have been used in an attempt to attenuate gear noise originating within the reduction gear. Lead-core damping pads have been cemented to various panels of the enclosure structure which were found to vibrate with a high energy. Other sound deadening and absorbing material, such as glass fiber and acoustic cloth lining, has been applied to various panels of the enclosure structure. Inlet and outlet ventilation baffles and silencers have been used on all ventilation air flow paths.
The above-described prior art attempts using solidly-attached enclosures to reduce noise produced by a generator load package have not been entirely successful. In particular, the sound caused by vibration of the diaphragm-like steel base plate and the 120 Hertz and approximately 1,000 Hertz noise spikes have not been satisfactorily eliminated.